ignition/ecu/fuel pump electrical question!

[WhiteBiatchTSI]

Could one of you 4g63 jedi guys please explain the sequence of events (signal paths if you will) of varios interlinked systems upon starting the car. Specifically-crank/cas sensors, ignition power transistor and coilpack, fuel pump and the ECU. What sends power to what first? What information does the ECU process before allowing the fuel pump to power up (if thats what happens in that order!)?
Im dealing with the no spark- no fuel problem and have performed a pleathora of tests on various components with multimeter. A bad ground is becoming very likely, but I doubt that my car will let me off that eisily. I know this is a pain in the ass question, but hey, I own a DSM!!!

 

[Supa Dexta]

I don't know everything about it, but here's a bit.

The gold box under the dash, under the stereo is the MPI relay, it turns on the fuel pumps, and supplies power to sensors, coils, etc. The starter relay is a different one seen here:

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When turning the key from acc to crank, there is 2 relays, one that cuts power to the lights and accessories so that all power is there for cranking, (that is the first half of the key turn) on the last half is when the relay under the stereo clicks, and that activates the fuel pump, coils, and starter I believe.

Now we've been having a discussion on the link boards lately about how the pumps don't fire up until the ecu receives a signal from the cas, that the engine is turning. But Ive done the tests, unplug the signal wire from the starter and turn the key and the pumps do fire. As for what the ecu is doing during all this, I don't know that deep into it.

If you just turn your key to acc and go to the engine bay and jump a small wire from batt+ to the signal wire on the starter (small black wire, white strip I think, remove it and touch the contact on the starter) and turn the car over; you can test for spark, unplug a spark wire and put the plug in it, leaned against some metal, and watch it while you give it a quick turn over. Also like mentioned if you leave that singal wire off, you can turn the key ahead and listen for the fuel pump by turning the key to crank.

 

[yokotabrat]

Supa Dexta has a good start for you there!

One clairification on the fuel pump. You are correct it is provided power when the ecu sees a signal from the CAS, however it is also powered when the starter relay is engaged. So the sequence goes, fuel pump gets power when the starter relay is engaged when the key goes to start, then a second or so later, the ecu sees a good cas signal and pulls a pin low to keep the portion of the MPI relay that powers the fuel pump active.

If you've got time, the DSM tech manual goes into detail about everything you could ever want to know: Listing of free DSM CD files

Start with the fuel first since it should be easiest. Does it ever get power? Do you know about the fuel pump check connector against the firewall? You can provide power to this connector to turn the fuel pump on manually. Unhook the line that goes from the regulator to the return line and hook up your own hose while providing power to the fuel pump check connector to see if you get fuel.

Also, check your MPI fusible link. It should be a 20A fuse in a small box of 4 total fusible links.

 

[WhiteBiatchTSI]

Can I test the fuel pump while removed from vehicle?

 

[WhiteBiatchTSI]

forgot to mention-- before removing fuel pump, checked fuses for continuity, performed manual fuel pump test at firewall and tried to start with direct ground from the battery- no sound, no fuel. So I removed it. visibly, the pump looks very caked with buildup but screen looks ok. oh yeah, broke two studs upon removal but will worry about that later---WTF